This invention relates to pre-expanded particles of a polypropylene resin suitable for the production of thermal insulation materials, cushioning packaging materials, returnable delivery boxes, automobile bumper cores and so on, and to a method of manufacturing polypropylene resin in-mold foamed articles from the pre-expanded particles.
Polypropylene in-mold foaming products are superior in chemical resistance, heat resistance and distortion restoration rate after compression as compared to polystyrene in-mold foaming products, and are superior in heat resistance and compression strength as compared to polyethylene in-mold foaming products. For these reasons they are widely used as cushioning packaging materials, returnable delivery boxes, automobile bumper cores, and others.
In recent years, efforts are being made in a wide range of fields for the purpose of improving productivity by shortening molding time in production so as to lower the cost of industrial products. There is a strong demand to shorten molding time also in the manufacture of polypropylene in-mold foamed articles to reduce costs. In the field of packaging materials, moreover, higher expansion ratios of foamed articles are being achieved, thereby reducing manufacturing costs.
Various techniques for shortening the molding time are known in the manufacture of in-mold foamed articles from polypropylene pre-expanded particles. For example, Japanese Patent Publication Kokai No. 61-103944 discloses expanded particles of a non-crosslinked polypropylene having a density of 8 to 100 g/liter and a particle diameter of 1 to 5 mm and containing 0.02 to 1 part by weight of organic sodium phosphate. However, in working examples thereof, when performing in-mold molding of the polypropylene expanded particles obtained, the particles are compressed to 50%, constituting a production disadvantage. Moreover, the density of most of the obtained polypropylene molded articles is as high as 40 g/liter or more, and no satisfactory molded article having a low density below 25 g/liter is produced. The only low density molded article having a density of 20 g/liter obtained therein using an ethylene-propylene random copolymer is inferior in surface properties. Japanese Patent Publication Kokai No. 8-20662 discloses a method wherein polypropylene expanded particles exhibiting a high temperature fusion peak, the heat of fusion of which is more than 3.5 cal/g and is not more than 6.0 cal/g, are compressed and filled in a mold in a compression ratio of 10 to 60% and heated with steam to produce a molded article. This method, however, is limited to only a compressive filling molding conducted at comparatively high compression ratios, and most of the obtained foams have a high density of 40 g/liter or more.
It is an object of the present invention is to provide pre-expanded particles of a polypropylene resin which can be molded in a old in a shortened molding time to give cellular molded articles having a low density and excellent properties such as surface properties, dimensional stability and weldability of particles.
A further object of the present invention is to provide a method r producing polypropylene in-mold foamed articles having a low density, excellent surface appearance, an excellent weldability of expanded particles and other excellent properties in a shortened molding time with a minimized shrinkability.
These and other objects of the present invention will become apparent from the description hereinafter.
It has now been found that, by regulating three parameters, namely the weight per particle of polypropylene resin pre-expanded particles, the heat of fusion QH (quantity of heat found from fusion peak area in a fusion curve) for a peak on the higher temperature side of two fusion peaks measured by differential scanning calorimetry (DSC), and the average diameter of cells in the pre-expanded particles, so as to keep those parameters within specific ranges, the cooling time required in the in-mold foaming can be shortened and, moreover polypropylene in-mold foamed articles having a low density can be obtained without any problem.
In accordance with the present invention, there is provided a pre-expanded particle of a polypropylene resin having a weight of 0.3 to 1.8 mg per particle, an average cell diameter of 250 xcexcm or less, and a bulk density of 10 to 20 g/liter, wherein said pre-expanded particle shows two fusion peaks on a DSC curve when measured by differential scanning calorimetry (DSC) and the heat of fusion QH of the peak appearing on the higher temperature side is from 2.8 to 6.0 cal/g. The present invention also provides a method of manufacturing a polypropylene resin in-mold foamed article from such pre-expanded particles which comprises imparting a gas pressure of at least 1.5 atms to the pre-expanded particles by an inorganic gas, filling the particles in a mold capable of being closed but incapable of being hermetically sealed, and heating the particles with steam to fuse together, thereby giving a foamed article.
In accordance with another embodiment of the present invention, there is provided a pre-expanded particle of a polypropylene resin having a weight of 0.3 to 1.8 mg per particle and an average cell diameter of 250 xcexcm or less, wherein said pre-expanded particle show two fusion peaks on a DSC curve when measured by DSC, and the heat of fusion QH of the peak appearing on the higher temperature side is from 0.3 cal/g to less than 2.8 cal/g. The present invention further provides a method of manufacturing a polypropylene resin in-mold foamed article from such pre-expanded particles which comprises imparting a gas pressure of 1.18 to 1.5 atms to the pre-expanded particles by an inorganic gas, filling the particles in a mold capable of being closed but incapable of being hermetically sealed, and heating the particles with steam to fuse together, thereby giving a foamed article.